Carbon dioxide concentrations in the atmosphere are increasing every year. Each spring, CO2 concentrations drop as plants in the Northern Hemisphere turn green and begin photosynthesizing. Each fall, the amount of carbon dioxide in the atmosphere rises as plants shut down and soils keep respiring. On top of this annual cycle, each year CO2 concentrations drop a little less than they did the year before and rise a little bit more.

Over the past 50 years, CO2 concentrations have been increasing at an accelerating rate. In the 1960’s, the annual increase was less than 1 ppm per year. Over the past decade, CO2 concentrations have been increasing about 2 ppm per year.

The increasing CO2 concentrations in the atmosphere make me worry about grasslands. Grasses were forged under low CO2 concentrations. CO2 concentrations in late May of this year hit 400 ppm at the observatory in Hawaii where records have been kept since 1958. In early July, they were 397. They’ll probably dip down to 393 by September and then rise up to about 402 next May. You can see these patterns here.

Slowly and surely, year after year, the peaks and troughs creep higher and higher.

I worry about increasing CO2 concentrations for a number of reasons. One of those is that they might threaten grasslands throughout the world.

The threat isn’t direct. More CO2 helps plants grow better. But it helps some plants more than others.

The world’s 11,0000 grass species were forged under low CO2 concentrations. They show a number of adaptations that are especially good when CO2 concentrations are low. Many of them evolved C4 photosynthesis and they use silica to stiffen their cell walls. Both are good tricks for growing when CO2 concentrations are low.

Because grasslands expanded during these times of low CO2 concentrations, it stands to reason that there might be fewer grasslands when CO2 concentrations rise.

But who is favored more than grasses when there is more CO2 in the atmosphere?

Plants that don’t use C4 photosynthesis?

Plants that don’t use silica to build their walls?

Plants that use excess carbon to build carbon-intensive structures like wood?

Who is likely to be favored? Trees.

Trees need a lot of carbon in the air around them. There are no trees that use C4 photosynthesis. Few trees use silica to stiffen their walls. And wood is a luxury in a world where CO2 concentrations are low.

What does a grass look like when it is grown at low CO2 concentrations? Not that different than today. A tree? A shadow of itself.

Here’s how we know that. Scientists in South Africa decided to grow some of their Acacia trees at a range of CO2 concentrations. In South Africa, a lot of their grasslands are becoming invaded by trees. One of the hypotheses is that this could be associated with rising CO2 concentrations.

In the experiment, some trees were grown at the concentrations that were around when mammoths roamed the Earth. Others at modern concentrations. And some at higher concentrations than today.

What does a tree look like when it is grown at concentrations of the mammoths? Small. Future concentrations? Big.**

**This is Barney Kgope’s work. You pronounce his last name Kih-Go-Pay. Barney is pronounced just Barney. Despite the immense scientific resources the US has compared to South Africa, no one in the US has ever done anything like this with trees before. We don’t know what our cottonwoods or cedars or oaks look like when grown at low CO2 concentrations. You can see a picture of Barney’s work here and what invaded grasslands look like down there.

When we see how much trees are hurt by lower CO2 concentrations and how much they benefit by rising CO2 concentrations, the thought of trees taking over grasslands the world over is not too far-fetched.

We know that grasses and trees have battled for millions of years. When CO2 concentrations are high, forests gain an edge. When they are low, grasslands expand.

Today, grasses and trees continue to battle in grasslands around the world. Slowly, it looks like trees are winning more and more.

As CO2 concentrations go up, trees can just grow faster. They are no longer penalized by low CO2 concentrations. They can produce more wood to grow taller faster.

Fires are often used to keep trees out of grasslands. But, places that needed to be burned every 4 years to keep trees out, might need to be burned every two years. Dry places where trees or shrubs could never gain a foothold, now are seeing more and more trees and shrubs move in.

In all, rising CO2 concentrations help all plants grow faster, but some more than others. As we think about all the global changes that are happening, these direct effects of CO2 are a reason why grasslands are likely to be imperiled in many parts of the world.

It’s always been a battle to keep shrubs and trees out of grasslands.

That battle is intensifying.